Photochromic glasses, phototropic glasses, and reversibly darkening glasses, as such have been variously termed, had their genesis somewhat over twenty years ago. U.S. Pat. No. 3,208,860 described such glasses as exhibiting the capability of changing color (darkening) when exposed to actinic radiation (conventionally ultraviolet radiation), and of returning to their original color (fading to their original optical transmittance) when removed from the actinic radiation. That patent is directed generally to silicate base glass compositions wherein silver chloride and/or silver bromide and/or silver iodide crystals impart the photochromic effect to the glass. The preferred base compositions were located in the alkali metal aluminoborosilicate system.
Numerous patents have issued subsequently directed to specifically-defined areas of silicate glass compositions which demonstrated exceptional photochromic behavior and/or other unusual physical and/or chemical properties. To data, the most prevalent commercial use of photochromic glass has been in the field of ophthalmic lenses. As can be appreciated, glasses designed for that application must comply with a host of chemical and physical properties to meet industry-wide specifications. For example, the glasses must manifest melting and forming characteristics rendering them capable of mass producing lenses utilizing techniques conventional in the glass art. Also, the glasses must resist attack by acids and bases as required by the industry. The glasses must display various optical properties, e.g., refractive index, dispersion, excellent transparency, etc. Consequently, photochromic glasses destined for ophthalmic use have, perforce, been the result of a compromise between the phothchromic behavior demonstrated and the above-discussed matrix of chemical and physical properties.
In general, the latterly-issued patents have been drawn to narrowly-defined areas of silicate-based compositions which are asserted to exhibit superior photochromic properties. Customarily, the goal of those patents has been to devise glasses which would quickly darken to low transmittances upon exposure to actinic radiation and would rapidly fade to their original transmittance upon removal from the actinic radiation. The drive to increase the rate of fading has been the subject of much research.
The following are exemplary of such research:
U.S. Pat. No. 3,795,523 describes glasses consisting essentially, in weight percent, of 50-70% SiO.sub.2, 10-26% B.sub.2 O.sub.3, 6-14% Al.sub.2 O.sub.3, 5-15% alkali metal oxides including K.sub.2 O and Li.sub.2 O in the ratio Li.sub.2 O:K.sub.2 O of 1:01-1:07, with the sum of alkali metal oxides other than K.sub.2 O and Li.sub.2 O being less than the level of Li.sub.2 O, 0.005-0.03% CuO, 0.05-1% Ag, and Cl and/or Br and/or I being present in at least an amount sufficient to react stoichiometrically with the Ag. In one working example, a glass displaying an initial transmittance of 88% darkened to a transmittance of 52.2% after an exposure of 1.5 minutes to ultraviolet light, and returned to a transmittance of 88% after 3 minutes' removal from the light.
U.S. Pat. No. 3,833,511 discloses glasses consisting essentially, in weight percent, of 48-60% SiO.sub.2, 7-11% Al.sub.2 O.sub.3, 17-31% B.sub.2 O.sub.3, 10-16% K.sub.2 O, 0.5-5% BaO, with the ratio BaO:K.sub.2 O being 0.05-0.4, 0.15-0.7% Ag, and Cl and/or Br and/or I being present in at least an amount sufficient to react stoichiometrically with the Ag. In one working example, the glass darkened from an initial transmittance of about 92% to about 40% within about 2 minutes, and faded to its original transmittance in less than 5 minutes.
U.S. Pat. No. 3,998,647 is concerned with photochromic glasses refractive index corrected for ophthalmic applications consisting essentially, in weight percent, of
______________________________________ SiO.sub.2 48-62 BaO 0.5-7 B.sub.2 O.sub.3 15-22 BaO:R.sub.2 O 0.035-0.65 Al.sub.2 O.sub.3 0-7 TiO.sub.2 0-2 ZrO.sub.2 0-10 CuO 0.002-0.03 Al.sub.2 O.sub.3 + ZrO.sub.2 6-12 Ag 0.15-1 Li.sub.2 O +/or Na.sub.2 O +/or K.sub.2 O 6-16 ______________________________________
and wherein Cl and/or Br and/or I will be present in more than the stoichiometric amount to react with the Ag. In one working example, the glass darkened from an initial transmittance of about 92% to about 35% in 10 minutes and returned to about 90% of its original transmittance in no more than 10 minutes.
U.S. Pat. No. 4,001,019 presents photochromic glasses refractive index corrected for ophthalmic applications consisting essentially, in weight percent, of
______________________________________ SiO.sub.2 48-62 TiO.sub.2 0-10 Al.sub.2 O.sub.3 7-15 ZrO.sub.2 0-10 B.sub.2 O.sub.3 17-26 Nb.sub.2 O.sub.5 0-5 Li.sub.2 O +/or Na.sub.2 O +/ 6-16 TiO.sub.2 + ZrO.sub.2 + 0.5-15 or K.sub.2 O(R.sub.2 O) Nb.sub.2 O.sub.5 BaO 0.5-5 As.sub.2 O.sub.3 0-2 BaO:R.sub.2 O 0.05-0.4 Sb.sub.2 O.sub.3 0-2 AgO 0.15-1 As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 &lt;2 ______________________________________
and wherein Cl and/or Br and/or I will be included in at least an amount sufficient to react stoichiometrically with the Ag. After darkening, the glasses are asserted to return to about 90% of their original transmittance within 20 minutes.
U.S. Pat. No. 4,018,965 is directed to photochromic glasses refractive index corrected and chemically strengthenable for ophthalmic applications consisting essentially, in weight percent, of
______________________________________ SiO.sub.2 54-66 PbO 0-3 Al.sub.2 O.sub.3 7-15 Ag 0.1-1 B.sub.2 O.sub.3 10-25 Cl 0.1-1 Li.sub.2 O 0.5-4 Br 0-3 Na.sub.2 O 3.5-15 CuO 0.008-0.16 K.sub.2 O 0-10 F 0-2.5 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 ______________________________________
The glasses are averred to darken below 25% and demonstrate a fading rate such that, after 5 minutes' removal from actinic radiation, they will exhibit a transmittance 1.5 times the darkened transmittance.
U.S. Pat. No. 4,102,693 is drawn to photochromic glasses which may be refractive index corrected for ophthalmic applications consisting essentially, in weight percent, of
______________________________________ SiO.sub.2 31-59 ZrO.sub.2 0-7 Al.sub.2 O.sub.3 8-20 TiO.sub.2 0-5 B.sub.2 O.sub.3 18-28 PbO 0-7 Li.sub.2 O 0-3 Ag.sub.2 O 0.05-4 Na.sub.2 O 0-8 CuO 0-1 K.sub.2 O 0-16 Cl 0-04-0.5 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 Br 0-1 MgO 0-2.6 F 0-0.2 P.sub.2 O.sub.5 Cl + Br + F 0.13-1 ______________________________________
The glasses are stated to manifest a half fading time (the period required to fade from the darkened state to a condition in which one-half of the lost light transmission has been restored) of not more than one minute.
U.S. Pat. No. 4,130,437 claims photochromic glasses refractive index corrected and chemically strengthenable for ophthalmic applications consisting essentially, in weight percent, of
______________________________________ SiO.sub.2 54-66 PbO 0-1.25 Al.sub.2 O.sub.3 7-15 Ag 0.1-0.3 B.sub.2 O.sub.3 10-25 Cl 0.2-1 Li.sub.2 O 0.5-4 Br 0-0.3 Na.sub.2 O 3.5-15 CuO 0.002-0.02 K.sub.2 O 0-10 F 0-2.4 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 ______________________________________
At about room temperature the glasses will darken to a transmittance below 30% and will fade within five minutes to a transmittance at least 1.75 times that of the darkened transmittance after withdrawal from actinic radiation.
U.S. Pat. No. 4,190,451 defines photochromic glasses refractive index corrected and chemically strengthenable for ophthalmic applications consisting essentially, in weight percent, of
______________________________________ Li.sub.2 O 0-2.5 SiO.sub.2 20-65 Na.sub.2 O 0-9 CuO 0.004-0.02 K.sub.2 O 0-17 Ag 0.15-0.3 Cs.sub.2 O 0-6 Cl 0.1-0.25 Li.sub.2 O + Na.sub.2 O + K.sub.2 O + Cs.sub.2 O 9-20 Br 0.1-0.2 B.sub.2 O.sub.3 14-23 Ag:(Cl + Br) 0.65-0.95 Al.sub.2 O.sub.3 5-25 Mole ratio 0.55-0.85 R.sub.2 O:B.sub.2 O.sub.3 P.sub.2 O.sub.5 0-25 ______________________________________
Optional ingredients include 0-6% ZrO.sub.2, 0-3% TiO.sub.2, 0-0.5% PbO, 0-7% BaO, 0-4% CaO, 0-3% MgO, 0-6% Nb.sub.2 O.sub.5, and 0-4% La.sub.2 O.sub.3, the total of such optional additions not exceeding about 10%. At about room temperature the glasses will darken to a transmittance below 40% and will fade at least 30 percentage units of transmittance after 5 minutes' removal from actinic radiation.
U.S. Pat. No. 4,358,542 delineates photochromic glasses refractive index corrected and chemically strengthenable for ophthalmic applications consisting essentially, in weight percent, of
______________________________________ SiO.sub.2 55-60 PbO 0.1-0.25 Al.sub.2 O.sub.3 9-10 Ag 0.1-0.15 B.sub.2 O.sub.3 19-20.5 Cl 0.3-0.5 Li.sub.2 O 2-2.5 Br 0.05-0.15 Na.sub.2 O 2-3 CuO 0.0065-0.01 K.sub.2 O 6-7 ______________________________________
At about room temperature the glasses will darken to a transmittance below 25% and, when withdrawn from actinic radiation, will fade to a transmittance at least twice that of the darkened transmittance within five minutes.
U.S. Pat. No. 4,407,966 encompasses photochromic glasses refractive index corrected and chemically strengthenable for ophthalmic applications consisting essentially, in weight percent, of
______________________________________ SiO.sub.2 56-60 PbO 0.1-0.15 Al.sub.2 O.sub.3 6-9 Ag &gt;0.15-0.25 B.sub.2 O.sub.3 18-21 Cl 0.2-0.35 Li.sub.2 O &gt;2.5-3.5 Br 0.075-0.15 Na.sub.2 O 0.5-2.5 CuO 0.004-0.02 K.sub.2 O 5-7 CeO.sub.2 0-0.5 ZrO.sub.2 3.75-5 ______________________________________
At about room temperature the glasses will darken to a transmittance below 25% and within five minutes' removal from actinic radiation will fade at least 35 units of transmittance.
U.S. Pat. No. 4,550,087 reports photochromic glasses which, over the temperature range 0.degree.-40.degree. C., will darken to a transmittance between about 40-60%, will exhibit good temperature independence, and, over the temperature range 25.degree.-40.degree. C., will fade at least 30 percentage units of transmittance within five minutes' withdrawal from actinic radiation. Those glasses consist essentially, in weight percent, of
______________________________________ Li.sub.2 O 0-2.5 P.sub.2 O.sub.5 0-25 Na.sub.2 O 0-9 SiO.sub.2 20-65 K.sub.2 O 0-17 Ag 0.115-0.145 Cs.sub.2 O 0-6 Br 0.11-0.155 Li.sub.2 O + Na.sub.2 O + K.sub.2 O + Cs.sub.2 O 8-20 Cl 0.1-0.15 B.sub.2 O.sub.3 14-23 Cu.sub.2 O 0.007-0.011 Al.sub.2 O.sub.3 5-25 ______________________________________
wherein the molar ratio alkali metal oxide:B.sub.2 O.sub.3 =0.55-0.85, the weight ratio Ag:(Cl+Br)=0.65-0.95, and the sum of Ag+Br is at least 0.26%, when the Br content is 0.11%, and at least 0.25% when the Ag content is 0.115%. The refractive index suitable for ophthalmic applications can be secured by adding up to 10% total of the following ingredients in the indicated proportions of up to 6% ZrO.sub.2, up to 3% TiO.sub.2, up to 0.5% PbO, up to 7% BaO, up to 4% CaO, up to 3% MgO, up to 6% Nb.sub.2 O.sub.5, and up to 4% La.sub.2 O.sub.3.